Broadly, the invention relates to an improved apparatus for mixing dry particles with a liquid. More specifically, the invention is directed to an apparatus which is particularly suitable for mixing dry cement with water to obtain a cement slurry.
There are many chemical processes and other industrial applications which require mixing of dry solids with a liquid to obtain a working fluid or final product. To obtain satisfactory mixing of the solid and the liquid, the mixing device must meet two basic requirements. One requirement is that the device be capable of wetting the solids sufficiently to avoid forming agglomerates of the solid material. Secondly, the device must be able to furnish enough energy to thoroughly mix the solids and the liquid in a desired ratio.
As an example, well casings penetrating a subterranean formation are cemented with a slurry mixture obtained from mixing a dry cement blend with water. One of the conventional systems used to mix the cement slurry is a unit known as a vortex mixer. In one type of vortex mixer the dry cement particles are directed downwardly through a vertical pipe section which opens into a pump volute casing. As the cement drops into the volute casing it is contacted by water, the water being directed downwardly through an outer pipe section which encloses the "cement" pipe and which also opens into the volute casing.
In this mixer the volute casing and the lower end of the concentric pipe sections are supported inside a holding tank. As the slurry mixture leaves the pump volute, therefore, it is contained within the holding tank. From the holding tank, part of the slurry is continuously recycled through a densiometer and a recycling pump and back through the pump volute. At the same time, the remaining part of the slurry is directed from the recycling pump to a cement pumper and into the well bore.
The vortex mixer described above has certain disadvantages which make it unsuitable for mixing a dry solid and a liquid, such as cement and water. The main problem occurs at the outlet of the volute casing. The volute outlet is that point at which the slurry mixture passes from the volute casing into the mixture which is circulating in the holding tank. At the volute outlet the cement blend tends to form a mound of cement particles which stack up and obstruct the outlet.
This situation is caused by the fact that the water in the outer pipe and the cement in the inner pipe are moving along the same downward vector at the point of contact. To explain further, since the cement particles are much lighter than the water, the water is moving at a higher velocity than the cement at the point of contact. With both materials moving in the same directon, therefore, the drag friction between these materials is so low that the water is unable to sufficiently wet the dry particles.